Rare cells refer to cells present in very low concentrations, typically at less than 1000 cells per milliliter of bio-sample. Rare cells include viruses, prokaryotes, proteins, and such, present in bio-samples. For instance, in case of circulating tumor cells, the concentrations can be as low as 1 cell in 7 ml. Such rare cells in bio-samples may have to be captured for removal of the rare cells from the bio-samples or for detection of rare cells.